In a GSM network, signal transmission is performed between a mobile station (MS) and a Base Station Subsystem (BSS) comprising a Transcoder (TC) for encoding/decoding transmitted information and a Base Transceiver Station (BTS) for receiving/transmitting radio signals from/to the MS within a predetermined cell. Typically, the BTS and the TC are wire-connected using a synchronous PCM (Pulse Code Modulation) connection.
In a discontinuous transmission (DTX) mode, transmission is automatically interrupted in a speechless period to thereby reduce power consumption and interference of neighbouring cells.
During such speechless periods, Silence Descriptor frames (SID frames) comprising background noise information are transmitted. Thus, a receiver may bridge speechless periods by generating a noise signal (comfort noise) adapted to the background noise. In the GSM TDMA (Time Divisional Multiple Access) system, these SID frames are transmitted at reduced intervals (eg. every 480 ms) as compared to the usual GSM frame period during speech periods (4.615 ms in an air interface and 20 ms in the BSS).
A known DTX method of the synchronous PCM connection is described in the following.
According to the GSM system, a full rate speech frame (TRAU frame) consists of 40 octets (0-39) of 8 bit length, comprising speech parameter and flag information. Additionally, two octets of PCM synchronization information are provided at the beginning of the frame. The flag information of the TRAU frame comprises the following flags:
FLAGBITFRAME POSITIONMEANINGSID (LSB)C14bit 7, octet 3silence descriptor flagSID (MSB)C13bit 6, octet 3(two bit information)BFIC12bit 5, octet 3bad frame indication flagSPC16bit 7, octet 38speech flagTAFC15bit 8, octet 3time alignment flagDTXC17bit 8, octet 38discontinuous trans-mission flag
The SID, BFI and TAF flag only exist in an uplink TRAU frame and the SP flag only in a downlink TRAU frame, whereas the DTX flag exists in both TRAU frame types.
In the downlink direction, i.e. from the TC to the BTS, the TC sets a speech flag (SP flag) in the TRAU frame to thereby indicate whether the TRAU frame contains speech or not. In case of speech, the SP flag is set to “1”, otherweise to “0”. In the case of SP=0, the TRAU frame contains information used for generating comfort noise. The first TRAU frame containing SP=0 is transmitted to an air interface (radio transmitter) at the BTS and thereafter the transmission is stopped. However, the TC continues transmitting the comfort noise information to the radio transmitter at the BTS which transmits one frame every 480 ms. As soon as SP=1, continuous transmission is again started.
In an uplink direction, i.e. from the BTS to the TC, the BTS uses the BFI flag in the TRAU frame to indicate whether the TRAU frame contains a useful or a useless speech information, wherein BFI=1 indicates the useless speech information. The frame might have been modified during radio transmission such that the received speech parameters are destroyed.
When the TC receives a TRAU frame with BFI=1, the speech parameters of the current TRAU frame are replaced by the speech parameters of the previous TRAU frame. In case of several subsequent TRAU frames with BFI=1, a muting function is activated.
The SID flag of the TRAU frame consists of two bits and is used by the BTS to indicate whether the TRAU frame contains speech or not SID=00b (“b” indicates binary information) indicates that the TRAU frame contains speech information, whereas SID=10b indicates that information for generating comfort noise is contained. SID=01b indicates that the SID frame has been modified and that a previous one needs to be used.
Thus, if, for example, a TRAU frame with BFI=1 is received after a TRAU frame with SID=10b, the comfort noise generation is continued.
The TAF flag indicates whether the TRAU frame is aligned with a multiframe timing of a control channel (eg. SACCH, Slow Associated Control CHannel) to be used for exchanging control and measurement parameters between the MS and the BSS. In practice, during an uplink DTX, TAF=1 is in the same frame as SID=10b, because signaling is transmitted in the same air frame as the silence descriptor.
The DTX flag is assumed to be always set to “1”, since discontinuous transmission is always performed in both directions.
In ATM based GSM networks, the TC and the BTS are connected via an asynchronous ATM (Asynchronous Transfer Mode) connection. In an ATM connection, bit streams of binary signals of different channels are divided into unitary ATM cells to be transmitted in a time divisional manner.